Flash memory devices have undergone rapid development. Flash memory devices can store data for a considerably long time without powering, and have the advantages such as high integration level, fast access, easy erasing and rewriting. Flash memory devices have thus been widely used in different fields such as automation and control. To further improve the bit density and reduce cost, three-dimensional NAND flash memory devices have been developed.
A three-dimensional NAND flash memory device often includes a stack of gate electrodes arranged over a substrate, with a plurality of semiconductor channels through and intersecting the wordlines, into the substrate. The bottom gate electrodes function as bottom select gates. The top gate electrodes function as top select gates. The word lines/gate electrodes between the top select gate electrodes and the bottom gate electrodes function as wordlines. The intersection of a wordline and a semiconductor channel forms a memory cell. The top select gates are connected to wordlines for row selection, and the bottom select gates are connected to bitlines for column selection.